InRelease file: support Signed-By field

[LebedevRI]

https://wiki.debian.org/DebianRepository/Format#Signed-By says:

Signed-By
An optional field containing a comma separated list of
OpenPGP key fingerprints to be used for validating
the next Release file. The fingerprints must consist
only of hex digits and may not contain spaces.
The fingerprint specifies either the key the Release file
must be signed with or the key the signature key must be
a subkey of. The later match can be disabled by appending
an exclamation mark to the fingerprint.

If the field is present, a client should only accept future updates
to the repository that are signed with keys listed in the field.
The field should be ignored if the Valid-Until field is not present
or if it is expired.

For both the CLI tools and JSON, the field is taken as a string verbatim.

When specified, we must also provide Valid-Until field, and i'm not sure there is an 'infinity' value for it, so 100 years will have to do?

Fixes #1497

Description of the Change

This functionality is missing as compared to reprepro,
and i would have preferred not to have lost it when switching apt.bcachefs.org repo.
It's not terribly important, but hey, it's not like it's too intrusive to the users.

Checklist

• unit-test added (if change is algorithm)
• functional test added/updated (if change is functional)
• man page updated (if applicable)
• bash completion updated (if applicable)
• documentation updated
• author name in AUTHORS